The present invention relates generally to a method and apparatus for assisting in matching a pacer to a patient's heart, and more particularly to a system and method for automatically determining the minimum pacing pulse amplitude required to reliably stimulate cardiac contractions. The system and method are particularly well adapted for use in a pacer system analyzer, wherein the operation of a pacer is monitored in association with a patient's heart prior to implantation.
To assist physicians in treating cardiac disorders of the type for which the use of implantable cardiac pacers is indicated, pacer system analyzers (PSA's) have been developed. These devices are used at the time of pacer implantation to measure the parameters of a pacer system, which includes the patient's heart, the pacer to be implanted, and the previously implanted pacer leads, without the need to perform separate procedures requiring multiple interconnections and an undesirably long time to complete. Pacers to be implanted are tested for proper programming and operation, not only while connected in a simulated pacing system environment, but also while connected to the actual system in which they are to be used. Moreover, pacer system analyzers are preferably equipped to generate pacing pulses as required to support the patient during the pacer implantation process, independently of the pacer to be implanted.
By using a pacer system analyzer prior to implantation of a pacer, a physician is able to program the operating parameters of the pacer system as required to suit the specific needs of an individual patient before the pacer has been fully implanted and the implantation surgery completed. This minimizes the likelihood for inconvenient, costly, and potentially injurious explantation of the pacer and/or its associated pacer leads.
One important parameter of a pacer system is capture threshold, which represents the minimum pacer output energy level required to reliably stimulate cardiac contractions. This is typically determined by varying the strength and duration characteristics of applied pacing pulses while simultaneously monitoring intracardiac electrical impulses produced during each contraction of the patient's heart. Capture is indicated when each applied pacing pulse consistently results in the occurrence of a heart contraction.
Previously, the determination of capture threshold involved a relatively complex and time consuming procedure. A pacer system analyzer, having an adjustable internal pacer circuit, was first coupled to the heart through a conventional cardiac lead, and the pacing rate was adjusted to be above the patient's intrinsic rate. Then, the pulse energy was manually adjusted by the cardiologist. A monitoring device coupled to the lead provided a visual "sense" indication upon the occurrence of each naturally occuring cardiac contraction. Consistently induced contractions did not result in the production of the visual "sense" indication since the presence of substantial lead recovery artifacts made it necessary to design the monitoring device to be insensitive for a specified refractory period following each applied pacing pulse. Capture was indirectly presumed when the "sense" visual display ceased entirely, indicating that contractions were presumably occurring synchronously during the monitoring device's post-pulse refractory period.
In prior systems, such an indirect capture detection method was mandated by the presence of the post-pulse lead recovery artifacts, which result from depolarization of the interface between the pacer lead and cardiac tissue and which typically exceeds the level of evoked cardiac response signals by several orders of magnitude.
One prior technique for directly detecting cardiac response signals involved applying a post pacing pulse reverse current to the lead in order to more rapidly depolarize the lead-cardiac interface. This technique found application as a palliative, but as an "exact" lead depolarization technique suffered from the fact that the unequal charge-discharge time constants were non-linear functions of pacer drive level, lead type, lead geometry, and timemodulated lead impedance. In addition, owing to the anodic voltages required, this approach introduced the possibility of inducing lead deterioration through corrosion.
Another prior technique was based on the observation that lead recovery artifacts correspond generally to the exponential decay characteristics of a resistor-capacitor network, and involved computing the anti-log of the post-pulse signal to recover the induced cardiac response. The accuracy of this system was inherently limited by the degree to which lead recovery artifacts departed from simple exponentials.
The present invention is directed to a system for automatically measuring a patient's cardiac capture threshold. The measurement steps, including variation of generated pulse energy levels and detection of cardiac capture, are performed automatically, thereby improving accuracy and repeatability while saving operating-room time and reducing complexity. The system can be used with any cardiac lead configuration such as unipolar, bipolar, tripolar, etc., with any of the currently used lead materials such as platinum, elgiloy, etc., and with any of the lead tip geometries such as screw-in, ball-tip, parabolic, etc.
In view of the foregoing, it is a general object of the present invention to provide a new and improved system and method for determining the capture threshold of a patient's heart.
It is a further object of the present invention to provide a system and method for measuring capture threhsold in which the measurement is performed automatically.
It is still another object of the present invention to provide an automatic capture threshold determining system and method which directly detects induced cardiac responses.